Photochemistry of oxidized Hg(I) and Hg(II) species suggests missing mercury oxidation in the troposphere.

Abstract

Mercury (Hg), a global contaminant, is emitted mainly in its elemental form Hg⁰ to the atmosphere where it is oxidized to reactive Hg^II compounds, which efficiently deposit to surface ecosystems. Therefore, the chemical cycling between the elemental and oxidized Hg forms in the atmosphere determines the scale and geographical pattern of global Hg deposition. Recent advances in the photochemistry of gas-phase oxidized Hg^I and Hg^II species postulate their photodissociation back to Hg⁰ as a crucial step in the atmospheric Hg redox cycle. However, the significance of these photodissociation mechanisms on atmospheric Hg chemistry, lifetime, and surface deposition remains uncertain. Here we implement a comprehensive and quantitative mechanism of the photochemical and thermal atmospheric reactions between Hg⁰, Hg^I, and Hg^II species in a global model and evaluate the results against atmospheric Hg observations. We find that the photochemistry of Hg^I and Hg^II leads to insufficient Hg oxidation globally. The combined efficient photoreduction of Hg^I and Hg^II to Hg⁰ competes with thermal oxidation of Hg⁰, resulting in a large model overestimation of 99% of measured Hg⁰ and underestimation of 51% of oxidized Hg and ∼66% of Hg^II wet deposition. This in turn leads to a significant increase in the calculated global atmospheric Hg lifetime of 20 mo, which is unrealistically longer than the 3-6-mo range based on observed atmospheric Hg variability. These results show that the Hg^I and Hg^II photoreduction processes largely offset the efficiency of bromine-initiated Hg⁰ oxidation and reveal missing Hg oxidation processes in the troposphere.